Effect Of Setup Errors Research Articles

Quantifying the dosimetric effects of neck contour changes and setup errors on the spinal cord in patients with nasopharyngeal carcinoma: establishing a rapid estimation method

The purpose of this study was to quantify the effect of neck contour changes and setup errors on spinal cord (SC) doses during the treatment of nasopharyngeal carcinoma (NPC) and to establish a rapid dose estimation method. The setup errors and contour changes in 60 cone-beam computed tomography (CBCT) images of 10 NPC patients were analysed in different regions of the neck (C1–C3, C4–C5 and C6–C7). The actual delivered dose to the SC was calculated using the CBCT images, and univariate simulations were performed using the planning CT to evaluate the dose effects of each factor, and an index n}{}{mathrm{Dmax}}_{mathrm{displaced}} was introduced to estimate the SC dose. Compared with the planned dose, the mean (maximum) Dmax increases in the C1–C3, C4–C5 and C6–C7 regions of the SC were 2.1% (12.3%), 1.8% (8.2%) and 2.5% (9.2%), respectively. The simulation results showed that the effects of setup error in the C1–C3, C4–C5 and C6–C7 regions were 1.5% (9.7%), 0.9% (8.2%) and 1.3% (6.3%), respectively, and the effects of contour change were 0.4% (1.7%), 0.7% (2.5%) and 1.5% (4.9%), respectively. The linear regression model can be used to estimate the dose effect of contour changes (R2 > 0.975) and setup errors (R2 = 0.989). Setup errors may lead to a significant increase in the SC dose in some patients. This study established a rapid dose estimation method, which is of great significance for the daily dose evaluation and the adaptive re-planning trigger of the SC.

The effects of set-up errors on dose distribution in radiotherapy treatment for lung cancer

The effects of set-up errors on dose distribution in radiotherapy treatment for lung cancer

Effect of setup error in the single-isocenter technique on stereotactic radiosurgery for multiple brain metastases.

In conventional stereotactic radiosurgery (SRS), treatment of multiple brain metastases using multiple isocenters is time‐consuming resulting in long dose delivery times for patients. A single‐isocenter technique has been developed which enables the simultaneous irradiation of multiple targets at one isocenter. This technique requires accurate positioning of the patient to ensure optimal dose coverage. We evaluated the effect of six degrees of freedom (6DoF) setup errors in patient setups on SRS dose distributions for multiple brain metastases using a single‐isocenter technique. We used simulated spherical gross tumor volumes (GTVs) with diameters ranging from 1.0 to 3.0 cm. The distance from the isocenter to the target's center was varied from 0 to 15 cm. We created dose distributions so that each target was entirely covered by 100% of the prescribed dose. The target's position vectors were rotated from 0°–2.0° and translated from 0–1.0 mm with respect to the three axes in space. The reduction in dose coverage for the targets for each setup error was calculated and compared with zero setup error. The calculated margins for the GTV necessary to satisfy the tolerance values for loss of GTV coverage of 3% to 10% were defined as coverage‐based margins. In addition, the maximum isocenter to target distance for different 6DoF setup errors was calculated to satisfy the tolerance values. The dose coverage reduction and coverage‐based margins increased as the target diameter decreased, and the distance and 6DoF setup error increased. An increase in setup error when a single‐isocenter technique is used may increase the risk of missing the tumor; this risk increases with increasing distance from the isocenter and decreasing tumor size.

Secondary cancer risk after radiation therapy for breast cancer with different radiotherapy techniques

The aim of this study was to estimate the radiation-related secondary cancer risks in organs during the treatment of breast cancer with different radiotherapy techniques, such as three-dimensional conformal radiotherapy (3D-CRT), intensity modulated radiotherapy (IMRT), and volumetric modulated arc therapy (VMAT). The treatment plans for 26 patients with breast cancer who received whole-breast irradiation at a dose of 50 Gy included tangential field 3D-CRT with hard-wedges (W-TF), tangential field IMRT (2F-IMRT), multiple field IMRT (6F-IMRT), and double partial arcs (VMAT). Patients were divided into three groups according to the distance between the contralateral breast (CB) and the body of the sternum. Setup error was simulated by moving the isocenter, and the dose distribution was then recalculated without changing the field fluency distribution. Based on the linear-exponential, the plateau, and the full mechanistic dose-response models, the organ equivalent dose and excess absolute risk were calculated from dose-volume histograms to estimate the secondary cancer risks in organs. Compared with 3D-CRT, IMRT and VMAT showed excellent results regarding tumor conformity and homogeneity; however, the low dose volume to organs was considerably higher in 6F-IMRT and VMAT. Secondary cancer risks for 2F-IMRT were comparable or slightly lower than for W-TF, but considerably lower than for 6F-IMRT or VMAT. After setup error simulation, there was a small increase in secondary cancer risk for 2F-IMRT and an increase of 159% and 318% for 6F-IMRT and VMAT, respectively, compared with W-TF. Although these results were obtained in most patients, they did not necessarily apply to every individual. The secondary cancer risks in the CB decreased significantly in correlation with increased distance for all alternative techniques, although they were higher in VMAT and lower in 2F-IMRT regardless of the distance. After setup error simulation, the increased changes in secondary cancer risks in the CB were comparable between 2F-IMRT, 6F-IMRT, and VMAT, suggesting that the secondary cancer risks in the CB mainly depend on radiotherapy techniques and distance, although the effect of setup error cannot be ignored. In the contralateral lung (CL), the secondary cancer risks were almost independent from distance and depended mainly on radiotherapy techniques; they were rarely affected by setup error. VMAT was associated with a higher secondary cancer risk in the CL. For the ipsilateral lung (IL), the secondary cancer risks were higher than those in other organs because the IL receives high doses to achieve tumor control, and they were relatively lower in VMAT. This warrants special consideration when estimating the secondary cancer risk to the IL. The study results suggested that the optimal radiotherapy method for breast cancer should be determined on an individual basis and according to the balance between secondary cancer risks related to anatomic diversity and setup error, which can prevent blind selection of techniques.

Effects of the setup errors on dose distribution of target area and organs at risk for prostate cancer

Objective To evaluate the effect of setup errors upon the target area and the organs at risk (OAR) during radiotherapy for prostate cancer. Methods Twelve prostate cancer patients receiving treatment in the recent 1 year were randomly recruited in this study. The position of each patient was verified by using cone beam CT (CBCT) for 6-10 times during the treatment. In treatment planning system (TPS), the isocenter position was moved along the setup errors with averaging error value (Plan_A) and each CBCT value (Plan_F). The dose distribution was recalculated without changing the beam setting, weight factors and monitor units (MUs). The dose difference was statistically compared between the simulation and original plans (Plan_O). Results For clinical target volume (CTV) D95, there was a significant difference between Plan_A and Plan_O (P=0.008), whereas no significant difference was observed between Plan_F and Plan_O. There were significant differences between Plan_F and Plan_O, Plan_A and Plan_O (P=0.004, and 0.041) for the planned target volume (PTV) D95.For OAR, rectal V60, Dmax, left femoral V20, Dmax and right femoral Dmax significantly differed between Plan_F and Plan_O (P=0.026, 0.015, 0.041, 0.049, 0.003). However, only left femoral Dmax significantly differed between Plan_A and Plan_O (P=0.045). The movement in the superior-inferior (SI) direction was significantly correlated with the changes in the rectal V40, V50 and V60 and PTV D95(r=-0.785, -0.887, -0.833, 0.682). The movement in the anterior-posterior (AP) direction was significantly associated with the variations in the bladder V20, V30, V40, V50 and V60(r=-0.945, -0.823, -0.853, -0.818, -0.774). The evaluation indexes of all normal tissues in the re-plan could meet the clinical requirements. However, the volume of target prescription volume had different levels of deficit, and the deficit of Plan_F was greater than that of Plan_A. Conclusions The simulation results of averaging into the TPS underestimates the effect of daily setup errors on the dose distribution. The effect of setup errors on the dose distribution in target area is greater than that of normal tissues. Y-direction errors are more likely to cause the variations of the rectal and PTV dose, and the errors in the z-direction are inclined to cause the changes in the bladder dose. Key words: Prostate neoplasm/radiotherapy; Setup error; Dose distribution

7. Dosimetric effects of setup error, range and dose calculation uncertainties: comparison between Pencil Beam and Monte Carlo algorithm

Purpose (1) To compare the dosimetric effect of setup error and range uncertainties with the effect of using two dose calculation algorithms (Pencil Beam (PB) vs. Monte Carlo (MC)) on protontherapy treatment plans, and (2) To assess which uncertainties affect the most the difference between the nominal and actual dose distribution. Methods 12 treatment plans designed for clinical treatment at the Proton Therapy Centre in Trento were selected, covering four anatomical sites: brain, head and neck, chordoma of the spine, and craniospinal axis. The plans, which were initially optimized with the PB algorithm, were recalculated with MC with a statistical uncertainty of 1%. Then, we evaluated the effect of geometrical (setup) and range uncertainties on the planned dose distribution with an in–house software. This software simulates 8 setup errors and 2 range errors (16 calculations per plan in total), in order to estimate near worst-case scenarios for target volume and organs at risks (OAR). We evaluated the differences between MC vs. PB distribution and the robustness of PBS plans using the same dosimetric indices used to optimize the plan (e.g. D1 and D99 for the target volume D1 for serially responding complications, etc.). Results In Table 1 , we show CTV and OAR dosimetric indices for the plans optimized with PB, then recalculated with MC, and also the worst-case scenario after robustness analysis. CTV D99 in MC plans are smaller than in PBS plans. Furthermore, except for plans of intracranial treatments, MC dose distributions are in general worse than the worst-case scenario (Fig. 1). Consequently, sparing of OARs with MC is obtained. For instance, the lacrimal gland mean dose is lower by 9% and 50% in MC vs PB for the static case and the worst case scenario, respectively. Conclusions In 2 out of the 4 disease sites we studied, the effect of using two dose calculation algorithms is greater than the effect of setup error and range uncertainties especially in CTV coverage. Considering MC as a gold standard, the minimum dose to the CTV isn’t guaranteed when plans are designed with PB.

Effects of setup error on brainstem dose distribution in patients with nasopharyngeal carcinoma and weight loss

Objective To investigate the effects of setup error acquired by cone-beam computed tomography (CBCT) on the dose to the brainstem. Methods Setup errors were collected from 4 patients with locally advanced nasopharyngeal carcinoma (NPC) undergoing three-dimensional radiotherapy. In the 33 radiotherapy plans, the upper limit of the dose to the brainstem was 54 Gy in total radiation and 163.3 cGy in a single radiation. Treatment was re-planed using a corrected center based on setup error. For each patient, areas exposed to radiation higher than 163.64 cGy were recorded in each layer of the brainstem after single treatment. The CorelDraw software was used to superpose all overdose areas in each layer of the brainstem. The Photoshop software was used to calculate the volume of the overdose areas. The changes in weight after radiotherapy were recorded and analyzed using paired t test. Results In 116 CBCT scans without correction, the setup errors were (0.90±0.85), (0.98±0.6), and (0.98±1.05) mm in x, y, and z directions, and (0.25±0.49)° in rotation. According to the images of a total of 2 502 layers of the brainstem acquired by 116 CBCT scans, 2 040 layers of the brainstem had doses over the upper limit to different degrees. The patient with the highest frequency of overdose in the brainstem had a total overdose volume up to 3.83% in 20 scans. Another patient had the overdose volume only 0.13% of the total volume. There was no significant relationship between weight loss and setup error (P>0.05). Conclusions Setup error acquired by CBCT has a substantial impact on the dose to the brainstem. It is an effective approach for verification of target volume delineation. Key words: Tomography, X-ray, cone-beam; Brain stem dose; Set-up; Nasopharyngeal neoplasms/three-dimensional radiotherapy

Definition of the set-up error using 2D radiographic images and 3D computed tomography and dosimetric effect of set-up errors for head and neck/lung radiotherapy patients

Definition of the set-up error using 2D radiographic images and 3D computed tomography and dosimetric effect of set-up errors for head and neck/lung radiotherapy patients

Effect of the setup error on the dosimetric verification for the volumetric modulated arc therapy

Objective To evaluate the effect of the setup error on the dosimetric verification for the nasopharyngeal carcinoma patients and cervical carcinoma patients treated with volumetric modulated are therapy (VMAT).Methods VMAT plans for 10 cervical cancer patients and 10 nasopharyngeal carcinoma patients were transplanted into the Delta4 phantom and calculate the dose,next,implement the treatment on the Varian iX linear accelerator.on the Varian iX linear accelerator.To simulate the setup error by moving the treatment couch in,out,up,down,left,right by 3 mm,5 mm,7 mm.Thereby study the effect of the setup error on the pass rate of the dose verification.Results The results for the dose distribution using the gamma evaluation method showed that the pass rate (3％/3 mm) was less than 90％ when the setup error were greater than 3 mm and 5 mm for the nasopharyngeal carcinoma patients and the cervical carcinoma patients.The pass rate of head direction were (64.7 ± 8.2) ％ and (63.3 ± 3.6) ％ on setup error of 5 mm and 7 mm for nasopharyngeal carcinoma patients and cervical carcinoma patients,respectively.Conclusions Setup error has great effect on the dose verification of the VMAT plans,the greater of the setup error,the lower of the pass rate.The setup error of head direction is more sensitive than other directions especially. Key words: Volumetric modulated arc therapy; Setup error; Dosimetric verification

Evaluation of 3D fluoroscopic image generation from a single planar treatment image on patient data with a modified XCAT phantom

Accurate understanding and modeling of respiration-induced uncertainties is essential in image-guided radiotherapy. Explicit modeling of the overall lung motion and interaction among different organs promises to be a useful approach. Recently, preliminary studies on 3D fluoroscopic treatment imaging and tumor localization based on principal component analysis motion models and cost function optimization have shown encouraging results. However, the performance of this technique for varying breathing parameters and under realistic conditions remains unclear and thus warrants further investigation. In this work, we present a systematic evaluation of a 3D fluoroscopic image generation algorithm via two different approaches. In the first approach, the model's accuracy is tested for changing parameters for sinusoidal breathing. These parameters include changing respiratory motion amplitude, period and baseline shift. The effects of setup error, imaging noise and different tumor sizes are also examined. In the second approach, we test the model for anthropomorphic images obtained from a modified XCAT phantom. This set of experiments is important as all the underlying breathing parameters are simultaneously tested, as in realistic clinical conditions. Based on our simulation results for more than 250 s of breathing data for eight different lung patients, the overall tumor localization accuracies of the model in left–right, anterior–posterior and superior–inferior directions are 0.1 ± 0.1, 0.5 ± 0.5 and 0.8 ± 0.8 mm, respectively. 3D tumor centroid localization accuracy is 1.0 ± 0.9 mm.

Simulated Effects of Setup Errors on a Hybrid IMRT Technique for Whole Breast Treatment

Simulated Effects of Setup Errors on a Hybrid IMRT Technique for Whole Breast Treatment

Using non-daily cone-beam computed tomography reduce dosimetric effect of set-up errors in intensity-modulated radiotherapy for nasopharyngeal cancer

Objective To assess the impact of set-up correction on dosimetry using non-daily kilovolt cone-beam computed tomography (KVCBCT) for nasopharyngeal cancer patients treated with intensitymodulated radiotherapy (IMRT). Methods The mean shift values from 14 nasopharyngeal cancer patients received KVCBCT scans during the first 5 treatment fractions were calculated as prediction of systemic set-up errors and used for off-line correction at 1.5 mm threshold level. Presumed that the systemic errors can be corrected by moving couch without residual errors, the pre-correction set-up errors in the remaining fractions were the sum of actual set-up errors and predicted errors. The dosimetric effects of non-daily protocol were simulated in the planning system and analyzed with physical dose parameters in 14 IMRT plans. Results In 10 patients with predicted systemic errors ＞ 1.5 mm, target dose was reduced significantly. The mean reduction of GTV-D98 ( dose received by 98％ of the volume of GTV ), CTVnx-D95 ( dose received by 95％ of the volume of CTVnx ), CTV1 -D98 ( dose received by 98％ of the volume CTV1 ) were 3. 8 Gy ( Z =- 2. 81,P =0. 005 ) ,4. 8 Gy ( Z =- 1.96, P =0. 050 ), 1.0 Gy ( Z =- 2. 82, P =0. 005 ), respectively. The effect on dose to CTV2 was much less. After correction, mean 3D vector positioning errors was reduced from 3. 6mm to 2. 3 mm (t =2.00,P =0. 000). After correction, the dose led to increase in GTV-D98, CTVns-D95,CTV1-D95 was 3.8 Gy (t=-2. 70,P=0.007),5.0 Gy (t =-2. 15,P=0.030),0.9 Gy (Z=-2.80,P=0. 005 ) respectively, and reduced the dose deviation greater than 3％ or 5％ for organs at risk.Conclusion Non-daily KVCBCT correction reduced dosimetric effect of set-up errors in IMRT for nasopharyngeal cancer patients. Key words: Tomography, X-ray computed, cone-beam; Set-up errors; Nasopharyngeal neoplasms/intensity-modulated radiotherapy

Effects of Setup Errors and Shape Changes on Breast Radiotherapy

The purpose of the present study was to quantify the robustness of the dose distributions from three whole-breast radiotherapy (RT) techniques involving different levels of intensity modulation against whole patient setup inaccuracies and breast shape changes. For 19 patients (one computed tomography scan and five cone beam computed tomography scans each), three treatment plans were made (wedge, simple intensity-modulated RT [IMRT], and full IMRT). For each treatment plan, four dose distributions were calculated. The first dose distribution was the original plan. The other three included the effects of patient setup errors (rigid displacement of the bony anatomy) or breast errors (e.g., rotations and shape changes of the breast with respect to the bony anatomy), or both, and were obtained through deformable image registration and dose accumulation. Subsequently, the effects of the plan type and error sources on target volume coverage, mean lung dose, and excess dose were determined. Systematic errors of 1-2 mm and random errors of 2-3 mm (standard deviation) were observed for both patient- and breast-related errors. Planning techniques involving glancing fields (wedge and simple IMRT) were primarily affected by patient errors (∼6% loss of coverage near the dorsal field edge and ∼2% near the skin). In contrast, plan deterioration due to breast errors was primarily observed in planning techniques without glancing fields (full IMRT, ∼2% loss of coverage near the dorsal field edge and ∼4% near the skin). The influences of patient and breast errors on the dose distributions are comparable in magnitude for whole breast RT plans, including glancing open fields, rendering simple IMRT the preferred technique. Dose distributions from planning techniques without glancing open fields were more seriously affected by shape changes of the breast, demanding specific attention in partial breast planning techniques.

WE-A-BRA-02: A Novel Beam-Specific PTV Design to Account for Setup Error and Range Uncertainties for Scanning Beam Proton Therapy

Purpose: Conventional PTV is purely geometric concept that is inadequate for proton therapy since it does not take into account range errors that arise from CT number uncertainties and interplay effect of setup error and tissue heterogeneity. Especially for spot‐scanning proton therapy, we can't use the passively scattered proton planning approach, such as compensator smearing. This study investigates a novel method of designing PTV that guarantees adequate target coverage to CTV under such uncertainties. Methods and Materials: The CTV was expanded laterally from the axis of beam's eye view (BEV) to account for internal organ motion and setup error. Ray tracing algorithm is used to calculate range uncertainty at distal and proximal surfaces. Due to lateral setup error and organ motion, range uncertainties were analyzed within the lateral motion breadth to construct the maximum (distal) and minimum (proximal) range surfaces along each ray line. This is equivalent to compensator smearing. Subsequently, these water‐equivalent ranges were converted to physical margins using local density information along each ray line. Mathematical phantom was constructed and planned using conventional PTV and new beam‐specific PTV under single field uniform dose treatment technique. Dose distributions were compared under different simulated setup error and motion. Results: The plans using conventional PTV showed significant cold spots and hot spots in the presence of setup error and motion. The minimum percent dose of CTV coverage dropped from 96.7.9% to 38.0% when both setup error and motion is applied. However, plans using beam‐specific PTV only showed minimum dose drop from 96.9% to 87.4%. The resulted dose distribution showed significant advantage in covering the edge of the CTV using beam‐specific PTV. Conclusions: This preliminary study indicates the feasibility of proton beam‐specific PTV to guarantee adequate target coverage under anticipated setup error in heterogeneous human tissue.

Impact of setup error on integral dose in head and neck cancer treated with intensity modulated radiation therapy

e17056 Background: Intensity modulated radiation therapy (IMRT) has been widely used in the treatment of head and neck cancer and has the potential of escalating the dose with less morbidity. Integral dose is a concern in radiotherapy especially with IMRT as it may lead to an increase in second malignancies. Setup error is another important issue in radiotherapy which affects the final treatment outcome. In this study, the effect of setup error on non-target integral dose (NTID) for head and neck cancer treated with IMRT has been studied. Methods: Twelve patients of carcinoma of nasopharynx treated by IMRT with simultaneous integrated boost technique were included. The dose to the gross tumor volume (PTV70), CTV1 (PTV59.4) and CTV2 (PTV54) were planned for 70 Gy, 59.4 Gy and 54 Gy respectively in 33 fractions. All the patients were planned with nine equally spaced beams. The IMRT plan was generated in Eclipse treatment planning system using sliding window technique. Keeping it as the base plan, the patient setup error was simulated in the treatment planning system for 3 mm, 5 mm and 10 mm by shifting the isocenter in all three dimensions viz. anterior, posterior, superior, inferior, right lateral and left lateral. NTID was calculated from the following relation NTID = (non-target tissue volume X mean non-target tissue dose)/1000. Paired sample t-test was performed to compare NTID for different shifts with no shift (statistical significance: p < 0.05). Results: The mean NTID for no shift was 120.75 ± 22.85 (median: 116.59). The mean NTID with isocenter shift along the anterior direction were 119.87 ± 22.80, 119.24 ± 22.80, 117.97 ± 22.52 for 3 mm, 5 mm and 10 mm respectively. Similarly, the NTID for the posterior shifts were 121.59 ± 22.96, 122.08 ± 23.07 and 123.26 ± 23.34 for 3 mm, 5 mm and 10 mm respectively. The anterior and posterior setup error showed statistically significant difference (p < 0.0001) for NTID compared to no shift whereas lateral, superior and inferior setup errors were found to be statistically insignificant. Conclusions: Setup error is an important issue in radiotherapy. Our study shows that setup error along the posterior direction significantly increases NTID. Hence, the setup error along the posterior direction may increase the risk of second malignances in IMRT. No significant financial relationships to disclose.

SU‐GG‐T‐484: Optimization and Normalization of Hypofractionated Lung Treatment Plans

Purpose: For small solitary lesions in the lung, we have adopted a protocol for hypofractionated treatment using multiple conformal or IMRT beams. Since the lesion is near unit density and the surrounding lung tissue is low density, the loss of scatter in the lung tissue can reduce the dose to the PTV, when, in reality, the CTV or ITV may be adequately covered with margin. We have performed calculations using several ways to create the PTV and to normalize the treatment plan to provide adequate coverage of the CTV or ITV with margin while minimizing hot spots which result from normalizing to the low density PTV. Method and Materials: Gated CT scans were obtained on 2 patients and target volumes were drawn on axial series representing several individual phases of the breathing cycle. An ITV was then created and used for planning. In addition, a PTV was created by adding a clinically relevant margin around the ITV. The resulting treatment plan was then used to calculate the dose delivered to the CTV for each of the phases of the breathing cycle using different normalization methods. ITV coverage was also studied by moving the ITV inside the PTV to simulate setup error. Results: 2 patients were planned using the above mentioned technique. Properly selected normalization results in adequate CTV and ITV coverage through all phases of the breathing cycle. Effect of setup error also results in adequate ITV coverage as the high dose region moves with the ITV. Conclusion: Small isolated lesions in the lung can be treated with hypofractionation, but care should be taken in selecting the normalization technique to minimize the creation of unnecessary hotspots. Coverage of the near unit density lesions is not predicted by the low density PTV coverage.

Simulation of Dose to Surrounding Normal Structures in Tangential Breast Radiotherapy Due to Setup Error

Setup error plays a significant role in the final treatment outcome in radiotherapy. The effect of setup error on the planning target volume (PTV) and surrounding critical structures has been studied and the maximum allowed tolerance in setup error with minimal complications to the surrounding critical structure and acceptable tumor control probability is determined. Twelve patients were selected for this study after breast conservation surgery, wherein 8 patients were right-sided and 4 were left-sided breast. Tangential fields were placed on the 3-dimensional–computed tomography (3D-CT) dataset by isocentric technique and the dose to the PTV, ipsilateral lung (IL), contralateral lung (CLL), contralateral breast (CLB), heart, and liver were then computed from dose-volume histograms (DVHs). The planning isocenter was shifted for 3 and 10 mm in all 3 directions (X, Y, Z) to simulate the setup error encountered during treatment. Dosimetric studies were performed for each patient for PTV according to ICRU 50 guidelines: mean doses to PTV, IL, CLL, heart, CLB, liver, and percentage of lung volume that received a dose of 20 Gy or more (V20); percentage of heart volume that received a dose of 30 Gy or more (V30); and volume of liver that received a dose of 50 Gy or more (V50) were calculated for all of the above-mentioned isocenter shifts and compared to the results with zero isocenter shift. Simulation of different isocenter shifts in all 3 directions showed that the isocentric shifts along the posterior direction had a very significant effect on the dose to the heart, IL, CLL, and CLB, which was followed by the lateral direction. The setup error in isocenter should be strictly kept below 3 mm. The study shows that isocenter verification in the case of tangential fields should be performed to reduce future complications to adjacent normal tissues.

Measurement of Dynamic Angular Error of a Rotation Axis of a Universal Spindle on Integrated Machine Tools by Using the Ball Bar Method

An integrated machine tool is a type of multi-axes machine tools that has capabilities of both milling and turning. This paper presents a method to measure the dynamic angular motion accuracy of a rotation axis (B-axis) of a milling spindle under the synchronous motion by the B axis and linear axes. The present scheme identifies dynamic motion errors at the tool tip in radial and tangential directions by using the combination of ball bar measurements. The effect of setup errors is discussed in details. As case studies, the present scheme is applied to two different integrated machine tools, the one with a B-axis driven via a worm gear, and the other with a B-axis driven by a direct drive motor. The dynamic motion error is evaluated and its error sources are diagnosed on each machine.

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To monitor effects of tumor regression and setup error in IMRT head and neck treatment through dosimetric analysis of periodic CBCT scans. A patient undergoing reirradiation of a large exophytic nasopharyngeal recurrence using IMRT with spinal cord/brainstem blocks received 8 weekly cone beam CT (CBCT) scans over the treatment course using an on-board kilovoltage imager (Varian OBI). To analyze each CBCT scan, contours of the PTV and critical structures were first transferred from the simulation scan using image registration. The treatment isocenter was located in the CBCT scan by visually registering CBCT DRR’s with orthogonal kV radiographs taken prior to treatment. Treatment fields were then transferred to the CBCT scan and a dose calculation was performed. Results were compared to the original planning scan dose distribution to assess potential dosimetric effects of tumor reduction that may warrant replanning. All dose distributions compared were generated without inhomogeneity corrections or bolus due to limitations of CBCT image quality and scan diameter. To study the potential effects of tumor reduction for more typical spinal cord and brainstem doses, an alternate treatment plan was generated for the same patient without blocks and retrospectively applied to the CBCT data in the same way. Finally, to separate out the effects of setup error and consider missing tissue alone, the outer contours from selected CBCT scans with significant tumor reduction were transferred to the planning scan and both the blocked and unblocked treatment plans were recalculated and compared to their original dose distributions. PTV volume decreased dramatically from 606 cm3 to 336 cm3 over treatment. There were no changes in the CBCT dose distributions due to the missing tissue which warranted replanning however due to the mask becoming loose, the patient was nonetheless remasked and replanned approximately halfway through treatment. Dosimetric indicies were within 5% of planned values on the average except for the brainstem d05 which was approximately 10% lower than the planned value. The brainstem was primarily superior to the treatment field. The fluctuation of spinal cord d05 was higher than that of other structures (standard deviation of spinal cord d05 over all scans was 9.8% in the blocked plan and 8.5% in the unblocked plan) due to setup fluctuations associated with the loosening mask. PTV D95 did not change significantly over treatment. There were no significant increases in dose to the structures examined when setup error was removed as described above. Point doses were very sensitive to this procedure: small changes in registration when transferring beams could result in large fluctuations in minimum and maximum point doses therefore indices such as dmin and dmax were generally avoided. Setup error was primarily responsible for fluctuations from scan to scan in PTV and critical structure doses, especially for structures near high dose gradients such as the spinal cord. Missing tissue alone did not bring about changes significant enough to warrant replanning in this initial case for both the blocked and unblocked plans. Results from additional patients treated with this protocol will be presented as well.

A comparison of techniques for simulating set-up error and uncertainty in head and neck IMRT

We have compared four computational methods for quantifying the effect of set-up error and uncertainty on delivered doses to targets and organs at risk in the intensity modulated radiation therapy treatment of head and neck cancer. These four methods were direct simulation, simple convolution, plus two modified convolution approaches. Discrepancies of up to 20% in the equivalent uniform dose (EUD) between direct simulation and simple convolution were estimated for the relatively superficial parotid gland at a systematic set-up error of 6 mm standard deviation and a random uncertainty of 2 mm standard deviation. Truncated convolution agreed with direct simulation to within 6% for all situations studied. However, of the four methods, only direct simulation can quantify the range of outcomes (EUD) associated with a finite number of courses and fractions. Our results are particularly relevant to the design of dose escalation studies in head and neck cancer.